1. Field of the Invention
This invention relates to methods of treating preheater waste gases to remove sulfur oxide emissions and more particularly to removing sulfur oxides from preheater waste gases by inserting CaO, which is present as fines entrained in the off gas from the calcining furnace, into an upper stage of a multi-stage preheater to react with and reduce sulfur oxides, and in particular the SO.sub.2, present in the preheater off gases.
2. Brief Description of the Prior Art
Sulfur oxides are contaminants in waste gases from cement manufacturing processes. They are partly produced by the combustion of fuels which are the source of process heat. The primary source of sulfur oxides in the waste gas stream are from the volatilization of certain components in the cement raw meal, and in particular the pyrites present in the cement raw meal, which typically occurs in the preheating stage. Because sulfur oxides contribute to air pollution, their emission is being regulated in an increasingly stringent manner.
Because of the importance of removing sulfur oxides from industrial waste gases, a variety of techniques have been developed to treat such waste gases to reduce or eliminate the emission of sulfur oxides.
U.S. Pat. No. 4,102,982, to Weir, discloses a process for removing pollutants such as sulfur dioxide from stack gases by passing the waste gases through a defined flow path into which at least one selected liquid reagent is introduced. For removing acidic pollutants such as sulfur dioxide a scrubbing solution containing lime may be used. Weir discloses that the use of limestone in conventional wet scrubbers for removing sulfur dioxide from waste gases has resulted in a lower efficiency of removal.
U.S. Pat. No. 4,576,803, to Hegemann et al., discloses scrubbing flue gases with scrubbing solutions containing lime which may include calcium carbonate as well. However, Hegemann does not disclose maintaining a high efficiency in scrubbing flue gases with scrubbing solutions containing limestone.
U.S. Pat. No. 5,512,097, to Emmer, teaches a method of removing sulfur oxides from a waste gas stream, by passing it through a reverse jet scrubber in which a suspension of finely divided limestone slurry is used as an absorption medium. The limestone is taken from the material comminuted for the cement manufacturing step.
It would be advantageous to remove harmful sulfur oxides from waste gas emissions without having to utilize acqueous scrubbers or an extensive amount of scrubbing equipment.
Accordingly, a need has continued to exist for a simple method of removing sulfur oxides from a waste gas stream.
An object, therefore, of the present invention is to provide an efficient process for removing sulfur oxides from the emission stream of a cement manufacturing process utilizing a non-aqueous sulfur oxide removal agent.
Another object is to remove sulfur oxides from the waste gas stream of a cement manufacturing process using a byproduct from the process via a method that does not require the utilization of an extensive amount of additional equipment.
Other objects of the invention will become apparent from the description of the invention which follows.
In general, the foregoing and other objects will be carried out by providing a process of manufacturing cement having low sulfur emissions from cement raw meal in an apparatus including a hot or cold combustion air source, such as a clinkering furnace or a clinker cooler, and a source of CaO as producing during a calcination step in the process. In one preferred embodiment, the apparatus will consist of a clinkering furnace, preferably a separate calcining furnace, and a vertically positioned multi-stage preheater, or a preheater having multi-stage strings.
The clinkering furnace will have a feed end and a clinker discharge end and means for supplying fuel to the discharge end to establish a burning zone in the clinkering furnace. There is a riser duct at the feed end of the clinkering furnace for discharging combustion gas produced in the burning zone to the calcining furnace.
Likewise, the calcining furnace has a duct to discharge off gases containing CaO fines to the preheater. Also present is means to remove a portion of such off gases from the calcining furnace and to direct said portion to a separator means for separating CaO fines from the off gases and means to direct the separated CaO fines to an upper stage preheater (i.e., typically the first or second stages in a four stage preheater or the first three stages in a five stage preheater, wherein the "first" stage is that stage where material to be heat treated first enters the preheater, the "second" stage is the next stage through which material passes, and so on sequentially through the preheater) of the multi stage preheater wherein there is a reaction between the CaO fines and SO.sub.2 gases to form calcium sulfites, which are separated from the preheater gases and are eventually incorporated in the cement clinker.
The calcining furnace will have an inlet for preheated cement raw meal to be calcined, an inlet for fuel, an inlet for air for combustion and an outlet for spent combustion air and calcined cement raw meal.
Typically, there will be a gas solids separator flow connected to the calcining furnace having (i) an inlet for spent combustion air, having entrained therein calcined cement raw meal fines, flow connected to the outlet of the calcining furnace; (ii) an outlet for separated gas connected to the gas inlet of the lowest stage preheater, and (iii) an outlet for separated calcined cement raw meal flow connected to the feed end of the clinkering furnace. As indicated, there is also a riser duct which flow connects the feed end of the clinkering furnace to the inlet for air for combustion of the calcining furnace for discharging combustion gas from the clinkering furnace to the calcining furnace.
A clinker cooler may be incorporated in the apparatus and will have an inlet for clinkered cement, an inlet or inlets for cooling air, an outlet for cooling air and an outlet for the cooled clinker. The outlet for cooling air may be flow connected to the inlet for air of either the clinkering furnace, the calcining furnace and/or the multi-stage preheater .